1. Field of the Invention
The present invention relates to a process for separating and recovering valuable metals, and, more particularly, to a process for separating and recovering valuable metals contained in waste catalysts used in hydrodesulfurization of petroleum products, etc. With the subject process, few kinds of chemicals are used, waste water that causes environmental pollution is minimally discharged, and also no by-products are formed by means of simple steps.
2. Description of the Related Art
Materials produced by having molybdenum/cobalt-based catalyst metals supported on alumina-based carriers are normally used as desulfurization catalysts for petroleum products. In use of this catalyst, for example, for the desulfurization of heavy oil heavy metals such as vanadium, nickel and the like, contained in the heavy oil, are caused to adhere to the catalyst, as well as the activity of the catalyst being gradually lowered on account of iron, sulfur, phosphorous, etc. adhering to the catalyst. Thus, these effects require discharging the contaminated catalyst as a waste catalyst and replacing it with a new catalyst to carry out desulfurization. Direct dumping of such a waste catalyst, not only causes environmental pollution, but also is associated with the waste of valuable metals. Thus, petroleum refining companies contract with disposal dealers to deal with waste catalysts. The disposal dealers recover valuable metals such as vanadium, molybdenum and the like from the waste catalysts received.
Several methods have thus far been tried to separate and recover valuable metals contained in the waste catalyst. One of them is a method that includes separating and recovering valuable metals through the use of caustic soda. This process involves removing oil matter, sulfur, phosphorus, etc. that adhere to the waste catalyst by roasting the waste catalyst in advance, and subsequently leaching the resultant material with an aqueous caustic soda solution to separate and recover valuable metals. However, the use of sulfuric acid for neutralizing the caustic soda solution results in the generation of a large amount of Grauber salt (sodium sulfate) as a by-product and also results in leaving the sodium salt to the final stage.
In addition, the aluminum component of the waste catalyst is leached as sodium aluminate from a carrier for the catalyst material such as alumina or silica/alumina. As a result, the aluminum needs to be separated as aluminum hydroxide. Furthermore, this method also poses problems in that all of the steps are complicated. For example, the steps regarding a variety of treating chemicals such as caustic soda, sulfuric acid for neutralization, ammonium sulfate for salting out, and the like, as well as those additional steps as necessary for separation of the extracting chemicals added in excess, refinement of the resulting extracted solution containing valuable metals, and so forth.
As another method, the soda roasting method exists which involves roasting a mixture of waste catalyst and soda ash (sodium carbonate) to change the metals into soda salts, and then separating the metals as an aqueous solution of valuable metal components by leaching the soda salts with water. However, this method causes problems as well in that all of the steps are complicated requiring various chemicals including soda ash, ammonium chloride for salting out, hydrochloric acid for neutralization, and the like, thus leading to the formation of a large amount of various kinds of by-products. Therefore, a method is desired that does not require the use of different kinds of chemicals, has simple steps, and is also capable of reducing the amounts of by-products.